Built it, and something sounds wrong?
Built the box and something sounds wrong? Match the symptom below. Every row is sourced in this site's research notes: the box rows from the enclosure-design research, the noise, shut-off, fuse, turn-on, polarity and bass-boost rows from the amp makers' own manuals. This page is the quick reference distilled from that sourced material.
Not sure which one it is?
| Symptom | Likely cause | Fix |
|---|---|---|
| Buzzing/rattling from the box itself | Unbraced panel flex, or a loose T-nut/screw | Add bracing; retighten or replace T-nuts |
| Hissing/whooshing from the port at high volume ("chuffing") | Air moving too fast through the port for the power being demanded — past about 17 m/s (the Box Builder prints this for your build), often with the port below the 12–16 in²/ft³ area floor — or sharp unrounded port edges | Increase port area/diameter (re-solve length to hold tuning — use the Port Calculator); round over or flare the port opening's edges; as a last resort, turn gain down |
| Faint hissing around the driver itself at high SPL | Air leaking past the driver gasket or a seam | Re-seat/replace the driver gasket; recheck glue seams with a smoke test or soapy water test under pressure |
| Driver visibly over-excursing / bottoming out | Box tuned too high for the power applied, box undersized for the driver, or (for shared-chamber dual builds) one driver failed and the survivor inherited double the airspace | Verify box volume against the driver's actual T/S parameters; check both drivers if shared chamber; reduce gain |
| Sound is "loose"/"flabby," bass seems unfocused | Box too large for the driver's parameters | Recompute sealed volume via Small's formula, or reduce net volume with bracing or solid blocks glued inside — not polyfill, which makes a box act bigger, the wrong way here |
| Sound is "tight"/thin, lacks low-end extension | Box too small for the driver's parameters, OR (for sealed boxes only) legitimately fixable with polyfill if the shortfall is modest | See the polyfill note below; otherwise the box needs to be rebuilt larger |
| On the deepest notes the cone flaps, slaps or bottoms out — it moves a lot but little bass comes out (ported box) | Amp's subsonic filter is switched off or set too low, letting the driver try to reproduce frequencies well below the box's tuning, where the port no longer loads the cone — unloading below tuning is a commonly cited cause of ported subwoofer failure | Set the amp's subsonic filter at about 75–85% of the box's actual tuning frequency on a 24 dB/octave filter, closer to tuning (about 85–95% of it) on a 12 dB/octave one (see the Port Calculator tuning result); never run a ported box with the subsonic filter off or set far below tuning. A filter set well above that makes the sub go quiet below a certain note — that is the filter doing its job, not a fault |
| Distorted/crackling sound, or a burning smell from the amp or sub | Amplifier clipping (gain set too high for the input signal, forcing a distorted square-wave-like output) or an already-damaged/burnt voice coil | Set gain properly with an oscilloscope or a known clean test tone/DMM method, not by ear at max volume. For the meter method, unplug the speakers and raise the gain until the amp's speaker terminals read V = √(P × R) AC volts, where P is the amp's rated power at load R: 500 W into 2 Ω is √1000 = 31.6 V. Use the test tone level your amp's manual specifies — makers differ on it. If the smell/distortion persists at reasonable gain, the coil may already be damaged and the driver needs inspection or replacement. If it only happens at high volume and the lights dim with the bass, check the supply too: see the shut-off row in the second table below |
| Amp seems to run out of power / doesn't sound as loud as its rating suggests on some bass notes more than others | The load the amp sees changes with frequency. A ported box's impedance peaks twice, once below its tuning and once above it, and dips close to the coil's DC resistance right at tuning; a sealed box has one peak, at its resonance. At a peak the amp delivers far less current and power than its rating implies; at a ported box's tuning it delivers more. A normal electrical property of voice coils in boxes | This is expected behavior, not a fault — it's the amp's actual delivered power at that specific frequency, not a wiring or amp problem; verify wiring/final impedance is correct before assuming something is broken |
| Rattling that seems to come from the vehicle itself, not the box | License plate, interior trim panels, loose cargo/tools, or anything else in the vehicle resonating at frequencies your new system now produces that the factory system never reached | Isolate by playing a slow sine sweep and pausing at the rattling frequency — tighten the license plate, add foam/felt behind loose trim panels, and secure or remove loose cargo; this is extremely common after a first subwoofer install and is not a box or driver problem |
Polyfill note (sealed boxes only — not for ported boxes)
Loosely filling roughly half a sealed box's internal volume with polyester fiber fill (~1 lb/ft³) makes the trapped air behave more isothermally, raising the apparent internal volume by roughly 10–25% (small boxes at the high end). This is a genuine, physically real effect — but it's a fine-tuning tool, not a rescue for a grossly undersized box. It will not "double" a box's effective volume and does nothing useful in a box already at or above the driver's target volume.
Do not use polyfill near or inside a ported box's port — tuning in a ported design
comes from the port's air mass/velocity, and fill near the port disrupts that
mechanism.
Noise, shut-offs, fuses, turn-on pop and bass that sounds wrong
These rows come from the amp makers' own manuals (JL Audio, Rockford Fosgate, Kicker, DD Audio, MTX) and Jensen's engineering notes on hum. Indicator lights, low-voltage cutoffs and heat limits differ from one amp to the next, so your amp's manual is the final word. On grounding, follow your amp maker's instruction — a short ground to bare chassis metal near the amp — and add or upgrade the battery-negative-to-chassis cable, as JL Audio, Rockford Fosgate and Kicker all advise.
| Symptom | Likely cause | Test | Fix |
|---|---|---|---|
| Noise | |||
| Whine that rises and falls with engine speed | Alternator noise reaching the signal: a ground difference between the head unit and the amp pushing current through the RCA shield, noise on the supply, or the RCAs picking it up | With the engine running and the gain up, unplug the RCAs at the amp (or plug in a shorted RCA plug). Noise stays: the amp's power, ground or speaker wiring. Noise goes: it is coming in through the signal path | Route the RCAs away from power wiring; re-ground the head unit and the amp to clean bare metal, at one shared central point or at different points (Rockford); add a battery-negative-to-chassis cable; have the alternator and battery load-tested; turn the gain down if it is set too high |
| Constant hum or buzz, not tied to engine speed | A poor ground at the amp or the head unit, or a bad RCA connection | Check the amp and head-unit grounds and the RCA plugs; swap in a known-good RCA cable | Re-ground to clean bare metal; reroute or replace the RCAs |
| Hiss, or a ticking that follows the engine | Gain set too high, so the amp hears more of everything; for ticking, the ignition system | Turn the gain down; for ticking, have the plugs, plug wires and regulator checked | Set the gain by meter (the distortion row in the table above) |
| Noise only when the headlights, air conditioning, brake lights or another accessory is on | That accessory's wiring is coupling into the audio | With the amp's inputs shorted, switch the accessories on one at a time | Reroute the audio cables; re-ground; an in-line noise filter |
| Noise in the speakers even with the RCAs unplugged or shorted | The speaker cables are picking it up, or the noise is on the amp's power or ground | Connect a small test speaker on short leads at the amp: noise gone means the car's speaker cables | Route the speaker cables away from power wiring. Still noisy on the test speaker: run the amp from a separate small battery; noisy even then, the amp itself is at fault |
| Noise only with the antenna connected | A ground loop between the head unit and the antenna | Unplug the antenna at the head unit | An antenna ground-loop isolator |
| Noise still there, and a ground-loop isolator is the next thing to try | An isolator breaks the RCA shield's ground link with a transformer, and a cheap transformer brings problems of its own | Listen with and without it in the line | Fix the ground first (rows above) and use an isolator last. If the amp has differential (balanced) inputs, they are built to reject noise common to both conductors: check its manual |
| Amp goes into protect or shuts off | |||
| Protect light on at turn-on, or the amp plays only at low volume | A speaker wire shorted to the other wire or to the chassis, a load below the amp's minimum, or a speaker terminal grounded to the chassis | Disconnect the speaker wires at the amp and connect a test speaker on a different, short pair of wires; meter the wiring for a short; check the final load against the amp's minimum (the Wiring Calculator works it out) | Fix the short; rewire to a load the amp allows; never ground a speaker lead to the chassis. The light's colours differ by model: the manual says what each one means |
| Amp shuts off at volume, or its light flickers with the bass; headlights dim with the bass | The supply dipping below the amp's low-voltage cutoff (JL's HD1200/1 shuts off below 8.5 V): the charging system, an undersized power or ground wire, or poor connections | Have the battery and alternator tested; check every power and ground joint, and the factory battery-to-chassis and alternator-to-battery wires (often 10–6 AWG). A common DC voltmeter can miss dips that are very short (JL) | Fix the joints; upgrade the power and ground wire; above a 60 A main fuse, upgrade those factory wires to 4 AWG or larger (JL); repair the charging system. The Electrical Calculator's "Measure before you buy cable" card has the test |
| Amp gets hot, goes quiet, and comes back by itself after a while | Thermal protection: heat, too little ventilation, or a load below the amp's minimum | Feel the heat sink; check the clearance around it against the manual (makers print anything from 1 to 4 in); check the final load against the amp's minimum | Give it open air or a fan (JL, DD Audio), or move it out of the trunk (Kicker suggests the passenger compartment); correct the load. Read your amp's manual for its own limits — not another brand's figure |
| A mono amp with two RCA inputs runs hot or distorts (JL documents this for its HD1200/1) | Only one of the two inputs connected | Check that both inputs are fed | Feed both inputs, with an RCA Y-adapter if the source has one output |
| Fuses, turn-on and pop | |||
| Amp does not turn on (power light off) | A blown fuse, a loose or pinched wire at a terminal, no +12 V at the remote terminal, or a bad ground | Test each fuse out of its holder with a continuity meter; check for +12 V at the power and remote terminals; jump +12 V to the remote terminal; check the ground is on clean bare metal | Replace a fuse with the same value; tighten and re-strip terminals; if jumping the remote wakes it, fix the remote lead (a relay, or a better switched source); redo the ground |
| Fuse blows the moment it goes in | A short: the power wire pinched or chafed through at the firewall or chassis, or reversed polarity | Leave the fuse out; check the power wire along its run and wherever it passes through metal; check polarity at the amp | Repair the wire and fit a grommet where it passes through metal; correct the polarity; fit the fuse last |
| Fuse blows after playing for a while | A fuse below the amp's draw, a sagging supply that makes the amp draw more current, or a loose or corroded fuse holder heating up | Compare the fuse with the amp maker's value and the wire's capacity; test the fuse out of its holder with a continuity meter, and if it is good, inspect the holder and its terminals | Fuse at least at the maker's value and no higher than the wire's capacity (Kicker); fix the supply (the shut-off row above); clean, tighten or replace the holder |
| Fuse blown after a jump start | Jumper cables connected wrongly | Check the amp's fuses | Replace with the same value; connect jumper cables the right way round |
| Pop or thump when the amp turns on or off | A spike coming in through the input signal or through the remote lead; a long ground wire | Disconnect the RCAs and switch the amp on and off: pop gone means it comes in with the signal; then try a different 12 V source for the remote lead | A delay turn-on module on the remote lead; a relay fed from a different 12 V source; keep the ground under 3 ft (DD Audio) |
| Bass sounds wrong | |||
| Bass sounds thin, hollow or patchy, or gets louder when one speaker lead is swapped | The sub and the speakers it hands off to are out of step around the crossover frequency (polarity, or the sub sitting farther from you), or one driver is wired the wrong way round. MTX, DD Audio and Kicker all list wrong polarity as a cause of weak bass | Flip the amp's 0/180 polarity switch (some amps call it "phase"), or swap the sub's two leads, and play music you know; keep the position with the smoother, tighter bass (JL Audio, MTX). In a multi-sub build, check every driver is wired the same way round | Keep the better polarity and wire every driver the same way round: a reversed one works against the others (Rockford Fosgate). Redo the check after changing the crossover frequency or slope (JL Audio). The switch can only make up for one distance; a DSP's time delay corrects for the distance itself |
| Distortion, or the protect light, after turning bass boost or EQ up | Boost is power: +3 dB doubles what the amp must deliver (MTX) and +6 dB is 4 times. It also drives the cone harder at the frequency the boost is set to, and the gain you set before the boost now clips (Kicker) | Set the boost back to zero and play the same track at the same volume | Set the gain first, then the boost, then set the gain again (JL Audio, Kicker); cuts do not need it (JL Audio). Rockford Fosgate warns that high boost can cause over-excursion and damage. Cut peaks rather than boosting dips (Rockford Fosgate) |